1. Technical Field of the Invention
The present invention relates to a thermal image recording apparatus for recording an image onto a thermal recording material by heating a glaze formed by an arrangement of thermal recording dots in accordance with image data.
2. Description of the Related Art
Recently, thermal image recording which is conducted on a thermal recording material is used for recording an ultrasonic diagnostic image. The thermal image recording has advantages such as that a wet developing process is not required, that it is easy to handle, and that it can be suitably used in image processing. In recent years, therefore, the use of the thermal image recording is not limited to recording of a small-size image such as that in ultrasonic diagnosis, and application to image recording for medical diagnosis such as MRI diagnosis or X-ray diagnosis in which a large-size high-quality image is requested has been studied.
In thermal image recording, as well known in the art, a thermal head having a glaze in which thermal recording dots (recording dots) for heating a thermal recording layer of a thermal recording material (thermal film) to record an image are arranged in one direction is used, the glaze is slightly pressed against the thermal recording layer of the thermal film, and, in this state, the recording dots of the glaze are heated in accordance with an image to be recorded while the film and the glaze are relatively moved in a direction perpendicular to the arrangement direction of the recording dots, whereby the thermal recording layer of the thermal film is heated to record the image.
On the other hand, in the thermal film, a transparent film is used as a support, and the thermal recording layer is formed on one face of the support. The transparent film functioning as the support is made of, for example, polyethylene terephthalate (PET) (glass transition temperature Tg=69° C.). As the thermal recording layer, for example, useful is a thermal layer that is formed by applying an application solution containing an emulsion in which microcapsules containing at least a basic dye precursor, and a developer are dissolved in an organic solvent sparingly soluble or insoluble in water, and then emulsified and dispersed.
In thermal image recording, relating to the surface of the thermal film in which the transparent film made of an organic resin such as PET is used as a support, usually the thermal recording layer is pressurized and heated by the glaze of the thermal head, and at the same time the rear face of the film is subjected to transportation by a platen roller. In the thermal film in which a PET organic resin film is used as the support, therefore, a force in the contraction direction due to pressurization by the glaze acts on the surface of the thermal film heated to a high temperature, such as the temperature is higher than the glass transition temperature (Tg) of the PET film constituting the support, and a force in the expansion direction due to pulling by the platen roller acts on the rear face. As a result, there arises a problem in that the support of the thermal film is curled toward the pressurized and heated side, i.e. toward the thermal head. Such curling is produced also by a phenomenon that, when the emulsion of the thermal recording layer is dehydrated with heating by the glaze and the film in this state is then cooled, the thermal recording layer is contracted by a degree corresponding to the insufficient water, and hence the recording surface to which heat has been applied is formed into a concaved shape.
In recent years, as described above, a large-size high-quality image is requested in an application such as MRI diagnosis or X-ray diagnosis. Therefore, curling, which has not caused a serious problem in a conventional small-size thermal film, becomes large in degree in accordance with the increased size of a thermal film, thereby causing the thermal film which has undergone thermal recording, to be difficult to handle. Usually, a thermal film is observed while being vertically hung in front of a light box. Consequently, such curling particularly causes the handlability and the visibility to be impaired.
Various improved techniques for reducing the degree of curling in a thermal film have been proposed. In a thermal printing method which uses an apparatus shown in FIG. 7, for example, a printing drum 5 is stopped with using a clamp 3 after a printed image is obtained. Therefore, an end portion in the running direction of a print sheet 7 which is not fixed to the drum 5 is downward moved under the influence of the gravity and the specific rigidity of the sheet, and hence makes contact with a protrusion end portion of a guide plate 9 which is disposed below the drum.
Then, the drum 5 is rotated in a direction opposite to that of the arrow 11, so that the sheet 7 is supplied to rollers 13. Via adequate guide plates, the sheet is thereafter supplied to rollers 15 which are rotated so as to attain the same transporting speed as the printing drum 5. At the same time, the clamp 3 is opened, and hence the sheet 7 is transported toward a heated roller 17.
The roller 17 is used for heating the rear face of the sheet 7 so as to offset a majority of the influence on the recording surface (front face) due to the heating by a printing head 19. Finally, the sheet 7 is supplied by rollers 21 onto a collection tray 25 via a slot opening 23 of a cover. In the figure, 27a, 27b, and 27c denote press rollers which are biased by respective springs.
According to the configuration, the rear face (opposite to the recording surface) of the print sheet 7 is uniformly heated, so that curling due to heating of the sheet 7 by the printing head 19 can be reduced. This is disclosed in JP-A-7-299921.
In the thermal printing method which uses the apparatus described above, however, the heat roller having a relatively large diameter, and the mechanism for inverting a thermal film must be disposed, and therefore the components for decurling increase the installation space, the power consumption, the production cost, and the noise level. The correction is conducted by winding the thermal film around the outer peripheral face of the heat roller. Therefore, the curvature for correction depends on the diameter of the heat roller, and the correction is conducted only at a constant bending amount. When the quantity of heat received by the film is increased by raising the heating temperature or lowering the transporting speed, a phenomenon that the temperature exceeds the development start temperature of the thermal film and the recorded image is darken, or so-called density fogging may occur. Moreover, the quantity of heat applied to the thermal film is varied depending on a recorded image. When a heat roller having a constant curvature is used, therefore, it is impossible to optimumly set the curvature for correction in accordance with a recorded image. As a result, optimum decurling corresponding to a recorded image cannot be conducted, and there arise the possibilities that curl remains, and that the quality of the recorded image is lowered by the decurling process.